WMTS Support addition

Contribution for WMTS 1.0.0 support in cesium JS.

The spec.js file (Specs/Scene/WMTSImageryProviderSpec.js ) still needs to be substantially filled.
Some tutorials or examples of use should also be added.
I know it's a mouthful, but this class should be called `WebMapTileServiceImageryProvider`.  We avoid acronyms and abbreviations in our public API.